Secret Places

Gain parameters are commonly used to model the performance of
both and transmit and receive antenna, but Antenna Factors are
normally only used to model the performance of transmit antenna
(which are more precise than receive only units).

While it may seem bizarre to specify AF for a receive antenna it
starts to make sense when you are using calibrated EMC/EMI antennas
for all TSCM procedures.

Of course Antenna Factors increase with frequency (as does
gain), and remember that Antenna Factor is different from
Transmit Antenna Factor (so be careful). (Antenna Factor is
calculated as AF = Electric Field/Voltage applied to Antenna and
is normally expressed in dB).

Remember that while Gain is commonly used to delineate the
directional response, AF will describe how "clean" the
antenna is. When performing TSCM we want the measurements to be
as "clean" as possible as we are working only slightly above the noise floor and have
to fight for even a fraction of a dB.

The high threat VHF band should be checked using an antenna
factor of at least 10 dB 1/m for signals between 20 MHz and 300
MHz. Antenna gain should be between unity and 4 dB over this
range. A bi-conical or discone works well in this band.

Antenna factors for frequencies above 300 MHz should be
between (at least) 15 and 20 dB 1/m. Antenna gain should be over
4-8 dB from 300 MHz to 1 GHz. Log periodic antenna work well in this band, but for
optimal results use a spiral log antenna.

At frequencies above 900 MHz it is desirable to use an
antenna which offers a factor of at least 25 dB 1/m, and 30 dB
1/m above 2 GHz. Antenna gain should be at least 8-10 dB above
900 MHz (a gain of at least 15 dB is preferred). A gain of at
least 15 dB is recommended between 1.2 GHz and 3 GHz.

From 3 GHz to 12 GHz an antenna factor of between 30 to 40 dB
1/m is good, but anything above 12 GHz calls for at least 40 dB
1/m of antenna factor, or it will be virtually impossible to
detect the signal. Antenna gain should be at least 8 dB over
this range. A gain of at least 20 dB is recommended between 3
GHz to 6 GHz, and at least 30 dB up to 12 GHz.

Above 12 GHz antenna factors of at least 35 dB 1/m must be
contemplated. A gain of at least 50 dB should be used when dealing
with frequencies in this range.

The antenna being used must be operated in both a horizontal
and vertically polarized position, and in a 45-degree
diagonal position for optimal signal detection.

Left and Right Handed Polarized signals should be checked
between 900 MHz and 12 GHz using an antenna factor of at least
25 dB 1/m and a gain of 2-4 dB. Remember that you will get a
polarization offset loss figure that must be taken into
consideration when using a linear polarized antenna.

The number of antenna azimuth and elevation positions used
will be directly related to the gain of the specific
antenna.

A good rule of thumb is that in a 10-meter by 10-meter square
room in an office environment to use at least 16 steps (22.5
degrees) along each axis for every 10 dB of antenna gain. For a
typical log periodic antenna (with 5 dB of gain) this would
lead to 64 measurement positions for each polarization for a
aggregate of 320 positions (assuming horizontal, vertical, diagonal,
LHCP, and RHCP polarization). Naturally, a small computer
controlled antenna positioning system is most helpful.

Here are some of the better solutions to antenna requirements that the author
personally uses during TSCM work:

A quality biconical antenna with balun and Delrin spacer blocks.
It has to have the impedance bar in the "birdcages," and avoid the
portable elements as you get poor results below 50 MHz. You will need
a really good LNA you can dedicate to this antenna, and the author would
recommend the Sonoma 310. The author would also recommend that you procure a
set of bandpass filters (with really low insertion loss) so you can
keep the Intermod to a bare minimum. The filters are important as the
low VHF TV channels and FM BCB can play havoc with the LNA. A
firewall 30 MHz highpass filter will also be helpful. This antenna
is best used from 88 MHz to 300 MHz. Remember that with a biconical
that you cannot have metal interfering with the pattern of the
elements (hence the Delrin spacers) http://www.emctest.com/productpage.cfm?model=3110B&producttype=Antennashttp://www.ara-inc.com/bicons.htmhttp://www.sonoma-instrument.com/pdf/310ds.pdfhttp://www.sonoma-instrument.com/pdf/317ds.pdf

Things start to get a bit dicey above 800 MHz (at least here in
the US). The big problems are the 800 MHz cellular bands, the pager
bands, and the 1.8 GHz PCS bands. With this in mind you will need a
dozen of so band pass filters, and a half dozen band reject filters.
You can use the discone to detect the presence of these signal, and the
spiral log to DF it. However, watch out for cellular/PCS phone being
used as an eavesdropping device (mind the remote channels). http://www.sonoma-instrument.com/pdf/317ds.pdf

On the extremely high threat 2.4 GHz and 5.8 GHz bands the author also prefers to use
a patch antenna (16-20 dB) to find the signal, a narrow band inter-digital filter, and a
32-54 dB LNA. This author then splits the signal and use half of it to drive a
90 dB RSSI indicator which makes it very easy to DF the offending
signals. The other half goes through a detector, and then to a pair
of headphones (or O'scope). The author can bypass the splitter and drop the
signal right to a SA if he need to. The author also uses a similar system without
the splitter to drive a signal into a microwave spectrum analyser. Once the
signal is located then, the author switches to calibrated antennas for the actual measurments.

Around 12.4 GHz things get a little touchy. The standard gain
horns will work, and so will the broadband horns, but a small tripod
mounted 24 inch parabolic reflector and small spiral log tend to be
more helpful up to about 40 GHz.

Above 2 GHz, but below 12.4 GHz you want about 45-70 dB of gain.
This breaks down to 16 dB from horn gain, and 40-54 dB from the
amplifier. Above 8.2 GHz you should try to stay with a dish system with
about 30 dB of gain, and an amplifier with at least 40 dB of low noise
gain (54 dB is a good number).

You may also find it handy to have a half dozen small antenna's
and some method of attaching them to things like edges of desks.
These are also great for hand-held use, and for general purpose
scanner antennas. The Radio Shack 20-006, and 20-023 make a good
combination, and you can attach it to a nice clamp you pick up at
Home Depot or Lowes.

If you don't yet use the Sonoma Instrument you should call them and
ask to speak with Vladan Temer, his phone number is (707) 542-8569.

For those of you not familiar with EMCO, they can be reached at:
(512) 531-6400, and this author would strongly encourage you to obtain several
of their printed catalogs.